RFR: 8317318: Serial: Change GenCollectedHeap to SerialHeap in whitebox [v2]

[Ivan Walulya]

On Sat, 30 Sep 2023 17:27:42 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Use more precise type for Serial GC. I also added a ` ShouldNotReachHere()` there, because using serial-heap when serial-gc is not used seems problematic.
>
> Albert Mingkun Yang has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains one commit:
> 
>   s1-prims

Marked as reviewed by iwalulya (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/15988#pullrequestreview-1661705568